Solar cells are photovoltaic devices which convert sunlight into electricity. Solar cells are made of crystalline silicon (c-Si) wafer based solar cells or thin film solar cells. Crystalline silicon solar cells are made from bulk materials cut into wafers, typically between 180 and 240 micrometers thick, which are then processed like traditional semiconductors. Thin film solar cell materials, for example amorphous silicon (a-Si) or copper indium gallium selenide (CIGS), are manufactured on a glass or plastic backing using vacuum processes including co-evaporation and sputtering. CIGS material strongly absorbs sunlight such that a much thinner film is required compared to a-Si or the traditional c-Si material. Because of this, a CIGS layer is thin enough to be applied to flexible substrates.
The solar power industry continuously strives to improve both the efficiency of the individual solar cell and of the overall solar module or array. A solar array is a collection of solar panels or solar modules wherein solar cells are linked together first in series to obtain the desired voltage, and then series strings are linked in parallel to produce more current. The solar module environment includes a power converter system which includes an inverter to convert the DC current into alternating current (AC) to power a home directly, or to be sent to the public power grid. The inverter may also transform and reshape the voltage to match the public power grid.
To maximize power output of a solar module, solar arrays use one of many different maximum power point tracking (MPPT) techniques. MPPT devices are typically integrated into a power converter system which provides voltage or current conversion, filtering, and regulation for driving various loads in power grids, and batteries. Individual solar cells have nonlinear output efficiency due to the relationship between solar irradiation, temperature, and total resistance. The maximum power point for a solar cell may be found by analyzing the curve of current to voltage. In general, the MPPT varies the system voltage to find the maximum power point for the cell or module measured. MPPT may be applied to the solar module, to solar sub-modules, or to solar cells directly. The more MPPT devices, the greater efficiency overall, but an increase in MPPT devices leads to an increase in system installation costs. Currently, the costs of installation at the solar cell level do not outweigh the benefits of the greater number of MPPT devices.
A pulse width modulator (PWM) may be used to control the amount of power delivered to a load while minimizing losses. PWM devices are integrated into the power converter system of the solar module. PWM functions by quickly switching the power on and off to reduce the power output. PWM with filters may also be used to condition the power output waveform to match the phase of the public power grid.